There
are two basic types of particle accelerator: circular and linear.

Circular
accelerators

In
a circular accelerator, the particles move in a circle until they reach sufficient
energy. The particle track is bent into a circle using dipole
magnets. The advantage of circular accelerators over linacs is that components
can be reused to accelerate the particles further, as the particle passes a given
point many times. However they suffer a disadvantage in that the particles emit
synchrotron radiation.

Synchrotron
radiation is more powerfully emitted by lighter particles, so these accelerators
are invariably electron accelerators. Consequently
particle physicists are increasingly using heavier particles such as protons
in their accelerators to get to higher energies. The downside is that these particles
are composites of quarks and gluons which makes analysing the
results of their interactions much more complicated.

The
earliest circular accelerators were cyclotrons, invented in 1929 by Ernest O. Lawrence.
Cyclotrons have a single pair of hollow 'D'-shaped plates to accelerate the particles
and a single dipole magnet to curve the
track of the particles. The particles are injected in the centre of the circular
machine and spiral outwards towards the circumference.

Cyclotrons
reach an energy limit because of the relativistic effects at high energies whereby
particles gain mass rather than speed. Though the special theory
of relativity precludes matter from traveling faster than the speed of light
in a vacuum, the particles in an accelerator normally travel very close to the
speed of light, perhaps 99.99%. In high energy accelerators, there is a diminishing
return in speed as the particle approaches the speed of light. The effect of the
energy injected using the electric fields is therefore to markedly increase their
mass rather than their speed. Doubling the energy might increase the speed a fraction
of a percent closer to that of light but the main effect is to increase the relativistic mass of
the particle.

Cyclotrons
no longer accelerate protons when they have reached an energy of about 10 million
electron volts, because
the protons get out of phase with the driving electric field. They continue to
spiral outward to larger redius but, as explained above, no longer gain enough
speed to complete the larger circle as quickly. There are ways for compensating
for this to some extent - namely the synchrocyclotron and
the isochronous cyclotron. They are nevertheless
useful for lower energy applications.

To
push the energies even higher - into billions of electron volts, it is necessary
to use a synchrotron. This is an accelerator
in which the particles are contained in a donut-shaped tube, called a storage ring. The tube has
many magnets distributed around it to focus the particles and curve their track
around the tube, and microwave cavities similarly distributed to accelerate them.

The
size of Lawrence's first cyclotron was a mere 4 inches in diameter. Fermilab
has a ring with a beam path of 4 miles. The largest ever built was the LEP at CERN with a diameter of
8.5 kilometers (circumference 26.6 km) which was an electron/positron collider. It has been
dismantled and the underground tunnel is being reused for a proton/proton collider
called the LHC due to start
operation in 2007.

The
aborted Superconducting
Supercollider in Texas would have had a circumference
of 87 km. Construction was started but it was subsequently abandoned well before
completion. Very large circular accelerators are invariably built in underground
tunnels a few metres wide to minimise the disruption and cost of building such
a structure on the surface, and to provide shielding against the intense synchrotron
radiation.

A
1960s vintage 2MV Van de Graaff linear accelerator
opened for maintenance

The
particles are accelerated in a straight line, with the target at the end of it.
Low energy accelerators such as cathode ray tubes and
X-ray generators use a single pair of electrodes with a dc
voltage of a few thousand volts between them. In an X-ray generator, the target
itself is one of the electrodes. DC accelerator types capable of causing nuclear
reactions are Cocroft-Waltons or voltage multipliers
that convert AC to high voltage DC and Van de Graaffs that use
static electricity carried by belts.

Higher
energy accelerators use a linear array of plates (or drift tubes) to which an
alternating high energy field is applied. As the particles approach a plate they
are accelerated towards it by an opposite polarity charge applied to the plate.
As they pass through a hole in the plate, the polarity is switched so that the
plate now repels them and they are now accelerated by it towards the next plate.
Normally a stream bunches of particles are accelerated, so a carefully controlled
AC voltage is applied to each plate to continuously repeat this for each bunch.

As
the particles approach the speed of light the switching rate of the electric fields
becomes so high that they operate at microwave frequencies, and so microwave
cavities are used in higher energy machines instead of simple plates.

High
energy linear accelerators are often called linacs.

Linear
accelerators are very widely used - every cathode ray tube contains
one, and they are also used to provide an initial low energy kick to particles
before they are injected into circular accelerators. They also can produce proton
beams, which can produce "proton-heavy" medical or research isotopes as opposed
to the "neutron-heavy" ones made in reactors.

Targets

The
output of a particle accelerator can generally be directed towards multiple lines
of experiments, one at a given time, by means of a deviating electromagnet. This makes
it possible to operate multiple experiments without needing to breach the void
to move tubes around.

Except
for synchrotron radiation sources, the purpose of an accelerator is to generate
high energy particles for interaction with matter.

This
is usually a fixed target, such as the phosphor coating on the back of the screen
in the case of a television tube, or a piece of uranium in an accelerator designed
as a neutron source, or a tungsten target for an X-ray generator. In a linac,
the target is simply fitted to the at the end of the accelerator. The particle
track in a cyclotron is a spiral outwards from the centre of the circular machine,
so the accelerated particles emerge from a fixed point as for a linear accelerator.

For
synchrotrons, the situation is more complex. Once the particles have been accelerated
to the desired energy, a fast acting dipole magnet is used to switch the particles
out of the circular synchrotron tube and towards the target.

A
variation commonly used for particle physics research
is a collider. Two circular synchrotons are built in close proximity -
usually on top of each other and using the same magnets (which are then of more
complicated design to accommodate both beam tubes). Bunches of particles travel
in opposite directions around the two accelerators and collide at intersections
between them. This doubles the energy of the collision compared to a fixed target
accelerator for a small increase in cost.

Higher
energies

At present
the highest energy accelerators are all circular colliders, but it is likely that
limits have been reached in respect of compensating for synchrotron radiation
losses, and the next generation will probably be linear accelerators five or ten
miles long. An example of such a next generation accelerator is the International
Linear Collider, due to be constructed between 2015-2020.

As
of 2005, it is believed that plasma wakefield accelerators in the form
of electron-beam 'afterburners' and standalone laser pulsers will provide dramatic
increases in efficiency within two to three decades. In plasma wakefield accelerators,
the beam cavity is filled with a plasma (rather than vacuum). A short pulse of
electrons or laser light eithers constitutes or immediately trails the particles
that are being accelerated. The pulse disrupts the plasma, causing the charged
particles in the plasma to integrate into and move toward the rear of the bunch
of particles that are being accelerated. This process transfer energy to the particle
bunch, accelerating it further, and continues as long as the pulse is coherent
(Matthew Early Wright. (April 2005). "Riding the Plasma Wave of the Future". Symmetry:
Dimensions of Particle Physics (Fermilab/SLAC), p. 12)

Energy
gradients as steep as 200 GeV/m have been achieved over millimeter-scale distances
using laser pulsers (Briezman, et al. [http://peaches.ph.utexas.edu/ifs/ifsreports/Self-focused762.pdf,
"Self-Focused Particle Beam Drivers for Plasma Wakefield Accelerators"]. Retrieved
13 May 2005) and gradients approaching 1 GeV/m are being produced on the multi-centimeter-scale
with electron-beam systems, in contrast to a limit of about 0.1 GeV/m for radiofrequency
acceleration alone. Existing electron accelerators such as SLAC could use electron-beam afterburners
to increase the intensity of their particle beams. Electron systems in general
can provide tightly collimated, reliable beams; laser systems may offer more power
and compactness. Thus, plasma wakefield accelerators could be used--if technical
issues can be resolved--to both increase the maximum energy of the largest accelerators
and to bring high energies into university laboratories and medical centers.

In
next few decades, the possibility of black hole production at the
highest energy accelerators may arise, if certain predictions of superstring theory
are accurate (Scientific American, May 2005). If they were produced, it is thought
that black holes would evaporate extremely quickly via Hawking radiation. However,
the existence of Hawking radiation is controversial. (Adam D. Helfer, "Do black
holes radiate?" Reports on Progress in Physics. Vol. 66 No. 6 (2003) pp. 943-1008
http://xxx.lanl.gov/abs/gr-qc/0304042
) It is also thought that an analogy between colliders and cosmic rays demonstrates
collider safety. If colliders can produce black holes, cosmic rays should have
been producing them for aeons, and they have yet to harm us. However, this is
also controversial. Models in which colliders cause trouble and cosmic rays do
not have been proposed.

Black
hole production would necessitate the development of new methods for investigating
in a terrestrial accelerator the kinds of extremely massive particles that are
thought to exist in dark matter and to have existed
during the Big Bang.